1. Field of the Invention
The present invention relates to a digital mobile communication apparatus using a time division duplex system and a code division multiple access system.
2. Description of the Related Art
In a time division duplex (TDD) system, the same frequency band is used in a reverse link (link from a mobile station to a base station) and in a forward link (link from a base station to a mobile station), called a ping-pong system, and a communication is performed alternately by dividing the time for a reverse link and for a forward link on the same radio frequency band. As shown in the thesis of xe2x80x9cMicro/Pico cellular Communication and Network Constructionxe2x80x9d written by Nakajima (The sixth Circuit and System; Karuizawa Workshop pp. 121-126, Apr. 19-20, 1993), it is known as a benefit of a TDD system that no space diversity is necessary in a mobile station, which results in the downsizing of a mobile station, because a transmission diversity can be applied in a base station.
A communication path propagated radio wave varies by the fading that is a factor to deteriorate the communication. As a technology to reduce the effect caused by the fading described above to achieve the high quality communication, there is reception diversity using more than two received waves. The reception diversity is one of the diversity technologies described above, and to obtain a plurality of fading received waves each having an independent fluctuation using more than two reception antenna separated away enough in a space. On the other hand, the transmission diversity is to estimate a communication path condition of a reception band for each antenna using the received wave in the reception diversity, and perform the transmission from an antenna estimated to be under the good communication condition. In a TDD system using the same frequency band for a reverse link and a forward link, the frequency characteristics depending on the fading fluctuation in a reverse link and a forward link is equal, and it is assumed that the Lime sameness property of fading fluctuations for a reverse link and a forward link is high when the switching time for both links are short enough. Because of It, by applying the transmission diversity, it is relatively easy to reduce the effect on a transmission wave caused by the fading fluctuation, which allows the high path communication quality for each channel.
As a conventional digital communication mode using a TDD system, PHS (Personal Handy PhoneSystem) that is a digital cordless phone system in Japan is known. PHS is a digital mobile communication system in a TDD system using a time division multiple access (TDMA) as a multiple access system. In a time division multiple access, the used time is divided in a radio frequency, the specific time slot is assigned to a user, and each user performs a communication in the assigned time slot.
FIG. 1 illustrates a exemplary frame configuration in a TDD system in a digital mobile communication system. A frame is composed of a reverse link burst and a forward link burst. Frame length 2500 is a sum of reverse link burst length 2510 and forward link burst length 2520. A reverse link burst is composed of reverse link communication control bits 2511, reverse link user information bits 2512, and reverse link error correction bits 2513. A forward link burst is composed of forward link communication control bits 2511, forward link user information bits 2512, and forward link error correction bits 2513.
Communication control bits are composed of guard time bits inserted to prevent a reverse link and a forward link from collision caused by propagation delay, unique word bits to acquire and hold the frame synchronization and so on. The number of reverse link communication control bits is referred to Cr, and the number of forward link communication control bits is referred to Cf.
User information bits include, for instance, information data of coded voice in a phone, and information data of coded image in a facsimile. The number of reverse link user information bits is referred to Ir, and the number of forward link user information its is referred to If.
Error correction bits are information bits to detect an information error caused on a transmission path and to correct the error so as to transmit information bits without an error in a communication link of low transmitted path communication quality. By inputting user information bits through an error correction coder, a series of error correction coded data (user information bits and error correction bits; however error correction coded data can not be separated distinctly into user information bits and error correction bits in convolution codes or the like) is generated. When an error occurs in error correction coded data in a transmission path, a reception side can estimate original user information bits using a pattern of received error correction coded data to reproduce. The number of reverse link error correction bits is referred to Fr, and the number of forward link error correction bits is referred to Ff.
In the conventional technology, reverse link burst length 2510 and forward link burst length 2520 are equal, and the numbers of communication control bits, user information bits or error correction bits in a reverse link and a forward link are also equal. Namely, Cr=Cf, Ir=If and Fr=Ff.
However, in a conventional digital mobile communication using a TDD system described above, the path communication quality difference between a reverse link and a forward link generates depending on the utilization environment. Because of it, for instance, in the case where the path communication quality of a forward link is lower than that of a reverse link, the problem occurs that the lower path communication quality of the forward link limits the entire path communication quality even when the path communication quality of the reverse link is high. Arid as described previously, in a TDD system, the space diversity is applied for the transmission and the reception in a base station by using the sameness property of transmission path condition of a reverse link and a forward link. In a reception diversity in the reverse link, since it is easy to perform the maximal-ratio combining of a received signal in each antenna branch, it is possible to obtain the large diversity effect. In the transmission diversity in the reverse link, a branch to transmit is selected when transmission phase correction is not performed. And even when the phase correction is performed, the effect of the transmission diversity is lower than that of the reception diversity because the transmission diversity is performed by assuming the transmission path condition. Accordingly, when the number of antenna set in a base station is increased, the path communication quality of the reverse link is largely improved, but the path communication quality of the forward link is not improved as largely as the reverse link. Generally in a CDMA system, since the path communication quality depends on the user capacity, the number of antennas in a base station may be determined according to the traffic of a cell, and it may occur that the difference of path communication quality between a reverse link and a forward link differs in cells.
The present invention is Lo solve the above problems. It is an object of the present invention to provide a digital mobile communication apparatus having high system capacity, which can improve the communication quality of a communication link of lower communication quality to achieve equal communication qualities of user information in the reverse link and the forward link equal.
In a digital mobile communication using a TDD system and a CDMA system in the present invention, in the case where the communication quality of the reverse link is higher than that of the forward link, the apparatus makes the error correction capability of the forward link higher than that of the reverse link by making a burst length of the forward link longer than that of the reverse link. And in the case where the communication quality of the forward link is higher than that of the reverse link, the apparatus makes the error correction capability of the reverse link higher than that of the forward link by making a burst length of the reverse link longer than that of the forward link. Thus, it is possible to improve the communication quality of the communication link of lower communication quality to achieve equal communication qualities of user information in the reverse link and the forward link equal, which results in the excellent digital mobile communication service with high system capacity.